DIY Audio Projects Forum

Amplifier Using BD139/BD140
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Author:  mwhouston [ 30 May 2016, 18:58 ]
Post subject:  Re: Amplifier Using BD139/BD140

BowToEd wrote:
I breadboarded this circuit and now think I want to pursue a better design.

The circuit was laid out on an IC breadboard and powered by a 12v gel cell battery. The midpoint voltage between the two output transistors was a little low but nothing I couldn't design out. Other than semiconductor substitutions, the rest of the circuit was pretty much stock.

I listened to some PMJ featuring such talents as Haley Reinhart and the magnificent Robyn Adele Anderson. I noticed some fuzz that was slightly annoying to me and it could hardly pass for quality audio.

I'm not going to give up on this design yet. It's designed for a supply of 18 to 22 volts and I was using 12.9 via a battery. If I use a stronger power source, I should be able to realize at least one watt of power.

All in all, the thing makes noise in a speaker. I think I'll end up making a different design before building something permanent, though.


If the amp only produces a few watts I'll assume you will be using efficient speakers. 90db efficiency is usually enough. My 330mW 1920s tube amp plays loud on 99db efficient speakers.

Author:  BowToEd [ 04 Jun 2016, 01:13 ]
Post subject:  Re: Amplifier Using BD139/BD140

The original design recommended a supply of 18v and promised 3w. I was using a 12v battery.

I tried a six transistor circuit on pg 76 of the same book. It's a very similar design except it uses a compound pair of transistors for the output amplifier. I used the same 3904/3906 for the base drivers and used a 4401 and 4403 for the input error amplifier and VAS.

Since I have a sleeping baby right now, I'm not running the thing at full power. It seems to be plenty sensitive and is free of distortion that the other one had. Robyn Adele Anderson sounds her usual pristine self and Lindsey Stirling made it all better with her magic fiddle made of gold. Maybe I'll run the thing at a higher power test when I can get the scope set up. I do need a better scope, though. Maybe someday.

It also doesn't help that I was using a crappy paper speaker I got from a thrift store for a dollar. Still, it's a test speaker and probably needs some sort of acoustic support. Maybe I'll make a plywood box and dress the amplifier up like a small tombstone radio.

The main problem is that most transistor amplifiers need far more than 12v to put much power into a speaker. Most power transformers with higher voltages lack the power to drive two amps to a decent level. Maybe I need to go home and rethink my life.


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